Heating - contact. Drawing, description

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For soldering carbide inserts on the holders of turning tools, local hardening of tools, and hard soldering of small parts, heating with high-frequency currents or a gas burner flame is used. However, HDTV equipment is bulky and expensive, it is not for a home workshop or a school circle. Only a highly qualified welder can use a gas burner in accordance with all the rules.

But there is another method of heating, almost forgotten in our time - on contact machines. It is most acceptable in the home, school, collective farm or state farm workshop, in a technical circle.

Its essence is the thermal effect of electric current on the conductor. The amount of heat released in this case depends on the magnitude of the current, the time of its action on the conductor and the electrical resistance of the latter.

Taking into account this dependence, we have developed an apparatus, which is a step-down transformer, the primary winding of which is designed for 220 V, the secondary - for 2 V. The cross-sectional area of the magnetic circuit is about 50 cm². The transformer is fixed on the base, and the contact tires of the secondary winding - on the insulator gasket.

The basis of the device is made of sheet steel 5 mm thick. Legs are screwed into it from below. Two windows are cut in the base: the smaller one is for ventilation, the larger one is for the exit of the bolts for fastening the contact busbars on the textolite insulator 10 mm thick. Holes along the edges of the insulator are used to fasten it to the base.

The ends of the secondary winding of the transformer are brought into the holes of the clamps and fixed with bolts (the grooves of the clamps allow them to be deformed during tightening and thereby ensure reliable electrical contact).

The transformer is covered with a protective casing and attached to the base with corners 25X25 mm. An insulating block is screwed to one of the upper corners - for connecting the wires of the primary overcasting with the power cord and the switch of the device.

Since amateur designers do not often have everything they need to repeat the described home-made work, we recommend starting the manufacture of the device with the selection of a package of transformer iron plates (preferably W-shaped), and only then start calculating the windings.

Let's assume that the cross-sectional area of your magnetic circuit (Q=a*b) is 36,8 cm². Then the power of the secondary winding of the transformer P₂ \u36,8d 36,8 * 1354,2 \uXNUMXd XNUMX W, and primary P₁ \u1354,2d 0,95 / 1425 \uXNUMXd XNUMX watts. Current I₁ = 1425 W / 220 V = 6,48 A; I₂ \u1354,2d 2 W / 677,6 V \uXNUMXd XNUMX A. We find the cross-sectional area of \uXNUMXb\uXNUMXbthe primary winding:

S₁= 6,48 A:2 A/mm² = 3,24 mm².

Wire diameter from here

d₁ \u4d 3,24 * 3,14 / 2 \uXNUMXd XNUMX mm.

Cross-sectional area of the secondary winding

S₂ = 677,6 A:2 A/mm² = 338,8 mm², and the wire diameter

d₂ \u4d 338,8 * 3,14 / 20,77 \uXNUMXd XNUMX mm.

Accordingly, we determine the number of turns:

n₁ = 220/000/222 = 36,8 turns, n₂ \u270d 2 * 220/2,5 \uXNUMXd XNUMX turns.

Rice. 1. Thermal heating apparatus (click to enlarge): 1 - protective cover, 2 - switch, 3 - power cord, 4 - base, 5 - insulator, 6 - contact bars, 7 - clamps, 8 - clamp bolts, 9 - leg ( 4 things.)

Rice. 2. Clamping lever (installed in an M6 threaded hole additionally drilled in the insulator) (click to enlarge): 1 - stand, 2 - lever, 3 - clamp

Heating - contact
Rice. 3. The contact section of the device: 1 - contact tires (red copper), 2 - clamping lever, 3 - carbide plate, 4 - solder, 5 - cutter holder, 6 - base insulator; A and B - heating zones

The efficiency and economy of the apparatus largely depend on the size of the plane of contact between the cutter body and contact bars. The amount of heat generated during the passage of an electric current depends on the resistance of the conductor at the points of contact. With a large plane of contact, little heat is generated with a significant consumption of electricity. Conversely, with a small plane, a lot of heat is released, which leads to instantaneous heating of the contacting layer. To avoid melting of the metal and breaking contact, the contact area is selected empirically.

When the cutter body is located on the tires, as shown in Figure 3, the heat generation center will be located in zone A; in zone B, a visually observed heat source does not occur due to the large contact surface.

Before soldering (including carbide inserts to cutter holders), it is necessary to perform a number of preparatory operations to ensure good solder flow and wettability of the parts to be joined. The supporting surface of the plates is ground and degreased. Tan also prepares the surface for the plate on the body of the cutter: it must be straight, without ledges and blockages along the edges. The protection of the surfaces of parts from oxidation during soldering is carried out by flux (brown).

Soldering occurs in this sequence. The holder of the cutter is placed on the tires of the apparatus. Between the surfaces to be joined, solder is placed with tweezers (a piece of brass sheet). For a more reliable pressing of the heated parts to each other and to the contact busbars, a lever mounted on an insulator plate is used. A stop is inserted into the groove of the lever, by which the carbide inserts are pressed against the cutter holders.

When the device is turned on, the contact zone quickly heats up, the metal melts, the contact is broken and the process is interrupted. There are two ways to avoid this: by operating in intermittent mode and by smoothly applying voltage to the windings. In the first case, the device turns on for 1,5-2 s, then turns off. At the moment of shutdown, heat is transferred from the point of contact along the body of the cutter without melting the metal.

Smooth voltage supply can be provided by LATR. By increasing the voltage, the same result is achieved as in the first case: heat is distributed in waves along the holder from the point of contact, providing heating to the melting temperature of the solder. The soldering process is controlled visually.

Such modes give the holder heating rate within 80-100 deg/s. This reduces internal stresses and prevents cracks in the carbide inserts. Slow cooling is necessary to avoid cracking in the solder joint.

A high-quality brazed seam should be no thicker than 0,1 mm. The length of non-soldered places should not exceed 10%.

In the absence of hard alloys, fragments of cutters, drills and other tools can be used as cutting inserts. The fragments are given the required shape on a grinding machine, or they are heated and forged to obtain a rectangular rod, which, when reheated, is cut with a chisel into separate plates.

Our device is multi-purpose. In addition to the manufacture of cutters, it can also be used for local hardening of tools (cores, chisels, screwdrivers, and so on). It is enough to touch the contact bars with the part of the tool that needs to be hardened and hold it for a few seconds. The heating temperature is controlled visually, by the color of the metal. At the same time, precautions must be observed: work in gloves and goggles on a grounded apparatus.

Authors: A.Bobrovnikov, V.Zinyuk

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